JPH06162596A - Optical head and optical information processor using the same - Google Patents

Abstract

PURPOSE:To provide a compact and high S/N optical head and a high- performance optical information processor. CONSTITUTION:Concerning the optical head for detecting the rotations of a polarizing plane of reflected light caused by the Kerr effect by irradiating a magneto-optical disk with line polarized light, a birefringent diffraction grating 3 for diffracting two orthogonally intersecting linear polarized light is arranged between a semiconductor laser 1 and a focus lens 4. The diffraction grating 4 diffracts and splits all the line polarized light orthogonal to the polarizing direction of an emitting beam 8 from the semiconductor laser 1. On the other hand, a phaser 13 for compensating phase difference is arranged on the optical path of reflected light. Therefore, since detected beams 10A and 10B provided with magneto-optical signal components can be splitted at a fine angle from the emitting beam 8 of the semiconductor laser 1 and the semiconductor laser 1 and photodetectors 7A and 7B can be adjacently arranged, the compact, high S/N and high-performance optical head and optical information processor can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a magneto-optical disk device,
The present invention relates to an optical head preferably used in an optical information processing device such as a magneto-optical card device or a magneto-optical tape device, and a compact and high-performance optical information processing device using the optical head.

[0002]

2. Description of the Related Art In general, the main structure of an optical head used in a magneto-optical disk device or the like has a perpendicularly magnetized film obtained by collecting a linearly polarized laser beam emitted from a semiconductor laser or the like with a focus lens. It consists of an optical system that irradiates the magneto-optical disk and an optical system that collects the reflected beam from the magneto-optical disk again with a focus lens and reflects it with a half mirror or a beam splitter to separate it from the optical path of the emitted beam of the semiconductor laser. ing. Since the polarization direction of the disc reflected beam is rotated (Kerr effect) depending on the magnetization direction recorded as information on the perpendicular magnetization film, the separated disc reflected beam is divided into two polarizations orthogonal to each other by the polarization separation element. After the components are separated, the two-division photodetector receives the light, and the magneto-optical signal is reproduced from the differential signal. However, since the rotation angle (Kerr rotation angle) of the polarization direction of the disc reflected beam is 1 degree or less, it is difficult to obtain a sufficient S / N ratio of the reproduced signal. Therefore, as described in Japanese Patent Publication No. 61-10888 (first known example), a s-polarized light beam and a p-polarized light beam are used as a beam splitter for separating the disk reflected beam from the optical path of the emitted beam of the semiconductor laser. A magneto-optical reproducing apparatus has been proposed which uses a beam splitter having different reflectances to increase the apparent Kerr rotation angle and improve the S / N ratio. As such a beam splitter, a laminated dielectric film sandwiched between two triangular prisms is generally used. Therefore, as shown in FIG. 1 of the first known example, the disc reflected beam is reflected and separated in the direction perpendicular to the optical path of the semiconductor laser emission beam. Then, according to the first known example,
Since the disc reflected beam has a structure in which it is reflected and separated in the direction perpendicular to the optical path of the emitted beam of the semiconductor laser, there is a drawback that the optical head becomes large, and this method cannot reduce the size of the optical head. was there. On the other hand, as disclosed in JP-A-3-178064 (second known example), a magneto-optical head using a quarter wavelength plate and a birefringence diffraction grating type hologram element has been proposed. . In the optical head device disclosed in the second known example, linearly polarized light emitted from a semiconductor laser or the like is transmitted without being diffracted by the hologram element of the birefringence diffraction grating type, and the quarter wavelength plate is transmitted. As a result, circularly polarized light is applied to the magneto-optical disk. Then, the polarization direction of the disc reflected beam is converted to a direction orthogonal to the polarization direction of the emitted beam of the semiconductor laser by reciprocating the quarter-wave plate, and then the birefringence diffraction grating type hologram element is used. This time it is all diffracted and received by the photodetector. In the action column of the second known example, "The signal reading method of the conventional magneto-optical disk utilizes that the plane of polarization of reflected light is rotated by the Kerr effect when linearly polarized light is incident on the disk. On the other hand, this method (second known example)
When circularly polarized light is incident, the phase of the reflected light changes due to the Kerr effect depending on whether the magnetization direction of the disk recording medium is upward or downward.
As described above, the light applied to the magneto-optical disk is circularly polarized light, indicating that a quarter wavelength plate is necessary. In addition, the birefringence diffraction grating type hologram element diffracts and separates only the polarization component orthogonal to the polarization direction of the semiconductor laser emission beam, and a diffraction grating serves as a diffraction grating for the same polarization component as the polarization direction of the emission beam of the semiconductor laser. It has no effect and is transparent.

[0003]

As described above, in the conventional magneto-optical disk device, as shown in the first known example, the method of irradiating the magneto-optical disk with linearly polarized light is mainly used. Therefore, the conventional magneto-optical disk can be used as it is as the reproducing method using the optical head, in which the magneto-optical disk is irradiated with linearly polarized light and the rotation of the polarization plane of the reflected light due to the Kerr effect is detected. Also, since a conventional magneto-optical signal reproducing circuit or the like can be used, compatibility is generated and it is convenient. However, when the disk reflected beam is reflected and separated in a direction perpendicular to the optical path of the emitted beam of the semiconductor laser by using a reflective separation element such as a half mirror or the beam splitter as in the first known example, the semiconductor laser is focused. In addition to the optical path of the focusing optical system reaching the lens, the optical path of the detecting optical system reaching the photodetector from the reflective separation element is required. Therefore, there is a problem that the optical head device becomes large and it is difficult to reduce the size of the optical head device.

An object of the present invention is to solve the above problems in the prior art. In an optical head for irradiating a magneto-optical disk with linearly polarized light and detecting the rotation of the plane of polarization of reflected light due to the Kerr effect, An optical head having a small size and a high S / N ratio, and a compact and high-performance optical information processing apparatus using the same, without having a structure for separating a disk reflected beam in a direction perpendicular to an optical path of an emission beam of a semiconductor laser. Is to provide

[0005]

In order to achieve the above-mentioned object of the present invention, as a first structure of an optical head of the present invention, a light source such as a semiconductor laser and a beam emitted from the light source are linearly polarized as a photomagnetic field. A focusing optical system including a focusing lens for focusing on a magneto-optical information medium such as a disc,
In an optical head including at least a diffraction grating that separates a reflected beam from a magneto-optical information medium from a beam emitted from a light source, and a photodetector that receives the beam diffracted and separated by the diffraction grating, the diffraction grating is incident. The separation ratio of the diffracted beam intensity to the beam intensity is set to be different for two orthogonal linearly polarized lights. As a second configuration of the optical head of the present invention, the diffraction grating in the first configuration has the first polarized light more than the first polarization separation ratio when the polarization direction is not rotated by the magneto-optical information medium. The separation ratio of the second polarized light whose polarization directions are orthogonal to each other is set to be larger. As a third configuration of the optical head of the present invention, the diffraction grating in the second configuration is configured to diffract and separate all of the second polarized light.
As a fourth configuration of the optical head of the present invention, the above first, second
Alternatively, the diffraction grating in the third structure is made of a birefringent material. Fifth of the optical head of the present invention
As the configuration, in the optical head according to the fourth configuration, an element for providing a phase difference is provided in the optical path of the reflected beam from the magneto-optical information medium or the separated beam by the diffraction grating. As a sixth structure of the present invention, any one of the optical heads shown in the above first to fifth structures is used.
By mounting a kind of optical head in an optical information processing apparatus, a small-sized, high-performance optical information processing apparatus having a high S / N ratio is realized.

[0006]

In the first structure of the optical head of the present invention, a linearly polarized laser beam is emitted from the light source and is narrowed down on a magneto-optical information medium such as a magneto-optical disk by an optical system for narrowing down the emitted beam. The polarization direction of the reflected beam from the magneto-optical information medium is rotated by the magnetization direction of the perpendicular magnetization film.
That is, a polarization component is generated in a direction perpendicular to the direction of the linearly polarized light irradiated on the magneto-optical information medium. The diffraction grating is
The two linearly polarized light components orthogonal to each other are diffracted and separated, and the two polarized light components diffracted and separated by the diffraction grating are combined to rotate the polarization direction. The diffracted and separated beam reaches a known magneto-optical signal detecting means including a polarization beam splitter, a two-division photo detector, etc., and can reproduce the magneto-optical signal.
In addition, the diffraction angle of the beam by the diffraction grating is
Since it is smaller than the reflection angle (right angle) by the beam splitter in the known example, the above-described known magneto-optical signal detecting means can be arranged near the light source, so that the optical head can be downsized. In the second configuration of the optical head of the present invention, the diffraction grating is generated because the polarization direction is rotated by the magneto-optical information medium rather than the separation ratio of the first polarized light when the polarization direction is not rotated by the magneto-optical information medium. Since the separation ratio of the second polarized light is larger, the apparent Kerr rotation angle of the diffraction-separated beam can be increased. Therefore, the S / N ratio of the magneto-optical signal can be further improved. In the third structure of the optical head of the present invention, the diffraction grating of the second structure can diffract and separate all the second polarized light generated due to the rotation of the polarization direction by the magneto-optical information medium. Thus, the apparent Kerr rotation angle of linearly polarized light of the beam diffracted and separated can be maximized, and the S / N ratio can be greatly improved. In a fourth configuration of the optical head of the present invention, the diffraction grating in the first, second and third configurations is made of a birefringent material, and the grating is used for ordinary rays and extraordinary rays. The difference in optical path length between the convex portion and the concave portion of the groove is made different. That is, it is possible to easily configure a diffraction grating in which the separation ratio of the diffracted beam intensity to the incident beam intensity differs depending on whether it is an ordinary ray or an extraordinary ray. Further, by setting the depth of the grating groove to an appropriate value, the diffraction grating having the first, second and third configurations of the optical head of the present invention can be manufactured. In a fifth structure of the optical head of the present invention, in the optical head using the diffraction grating of the fourth structure, an element for providing a phase difference in the optical path of the reflected beam by the magneto-optical information medium or the separated beam by the diffraction grating is provided. By arranging them, the phase difference generated in the substrate of the diffraction grating can be compensated, and the S / N ratio of the magneto-optical signal can be further improved. In the sixth configuration of the optical head of the present invention, the optical information processing apparatus is configured by using the optical head having a small size and a large S / N ratio, thereby realizing a small size and high performance optical information processing apparatus. be able to.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in more detail with reference to the drawings. FIG. 1 is a schematic diagram showing an example of the configuration when the optical head of the present invention is used in a magneto-optical disk device. In the figure, 1 is a semiconductor laser, 2 is a collimator lens, 3 is a diffraction grating, 4 is a focus lens,
5 is a magneto-optical disk, 6A and 6B are compound prisms, 7A
And 7B are photodetectors. The emitted beam 8 emitted from the semiconductor laser 1 becomes a parallel beam by the collimator lens 2, passes through the diffraction grating 3, and then passes through the focus lens 4
Thus, the magneto-optical disk 5 is narrowed down. The reflected beam 9 from the magneto-optical disk 5 becomes a parallel beam again by the focus lens 4 and is diffracted by the diffraction grating 3.
The detection beams 10A and 10B are separated from the optical path of the outgoing beam 8 and are condensed by the collimator lens 2. The detection beam 10A is split into a beam 11A and a beam 12A by the compound prism 6A and received by the photodetector 7A. Further, the detection beam 10B is generated by the composite prism 6
At B, the beam is separated into a beam 11B and a beam 12B, and is received by the photodetector 7B. First, the operation of the diffraction grating 3 will be described. The diffraction grating 3 is made of, for example, a material having birefringence such as lithium niobate (LiNbO ₃ ). LiNbO ₃ belongs to a trigonal crystal and is an optically negative uniaxial crystal, and its refractive index ellipsoid 1
As shown in FIG. 2A, 4 is flat in the direction of the optical axis (or the C axis of the crystal) 15. In the diffraction grating 3, for example, as shown in FIG. 2B, the optical axis 15 is in the plane of the diffraction grating 3, and the grating groove 16 of the diffraction grating 3 is formed on one surface. In this case, the cut surface 17 of the refractive index ellipsoid 14 is an ellipse whose minor axis is the direction of the optical axis 15. Therefore, the linearly polarized light 18 whose polarization direction is the optical axis 15 propagates as an extraordinary ray in the LiNbO ₃ crystal of the diffraction grating 3, and the linearly polarized light 19 whose polarization direction is orthogonal to the optical axis 15 is Propagate as an ordinary ray in the LiNbO ₃ crystal. Figure 3
As shown in (a), when the groove depth of the diffraction grating 21 made of the substance 20 having a refractive index n is d, the optical path length 22 of the convex portion is n.
At d, the optical path length 23 of the concave portion is d, and the difference ΔOP between the optical path lengths of the convex portion and the concave portion is ΔOP = (n−1) d. LiN
The birefringent materials such as bO _3, the ordinary refractive index n _o
And the refractive index n _{e of the} extraordinary ray are different, the difference ΔOP in the optical path length between the ordinary ray and the difference ΔOP in the optical path length between the extraordinary ray is different. The straight line 24 shown in FIG. 3B is L at a wavelength of 780 nm.
The optical path length difference ΔOP / λ in wavelength units obtained from the ordinary ray refractive index n _o = 2.262 of iNbO ₃ is shown, and the straight line 25 shows the ΔOP / λ obtained from the extraordinary ray refractive index n _e = 2.179. . The closer the value of the vertical axis is to a half integer, the stronger the action of the diffraction grating 21 is, and the intensity of the ± 1st-order diffracted beam is increased. Will be the diffraction grating 21. Further, the closer the value on the vertical axis is to an integer, the weaker the action of the diffraction grating, and the intensity of the ± first-order diffracted beams decreases. When the value is just an integer, there is no action of the diffraction grating 21 and only the zero-order beam is produced. Diffraction grating 21
When the groove depth d is 2.78 μm, the difference in optical path length between ordinary rays is 4.5 times the wavelength, and the difference in optical path length between extraordinary rays is 4.2 times the wavelength. Therefore, as shown in FIG. 2B, the diffraction grating 3 has linearly polarized light 18 (where the polarization direction is the optical axis 15).
Is an extraordinary ray), the 0th-order beam is strong and the ± 1st-order diffracted beam acts as a weak diffraction grating, and linearly polarized light 19 (the polarization direction is orthogonal to the optical axis 15 and becomes an ordinary ray).
, There is no 0th-order beam, and the ± 1st-order diffracted beams act as a strong diffraction grating. Next, reproduction of the magneto-optical signal will be described with reference to FIG. The arrow (polarization component) 26 in FIG. 4A represents the polarization component of the outgoing beam 8 of the semiconductor laser 1. The diffraction grating 3 is arranged so that the optical axis 15 coincides with the direction of the polarization component indicated by the arrow 26. Then, the outgoing beam 8 of the semiconductor laser 1 becomes an extraordinary ray when passing through the diffraction grating 3, for example, about 20% is diffracted to be ± 1st order diffracted beam, and about 70% is transmitted as a 0th order beam. The 0th-order beam is focused on the magneto-optical disk 5 by the focus lens 4,
The reflected beam 9 becomes a reflected beam 9. At this time, the reflected beam 9
The polarized light of is rotated by θk degrees depending on the magnetization direction of the perpendicular magnetization film of the magneto-optical disk 5, for example, as shown by an arrow 27 in FIG. The reflected beam 9 is again reflected by the diffraction grating 3
When incident on, the polarized light component 27 of the reflected beam 9 in the same direction as the polarized light component 26 is an extraordinary ray, so about 20% is diffracted like the polarized light component indicated by the arrow 28, and ± 1st-order diffraction is performed. Become a beam. On the other hand, of the polarized light components 27 of the reflected beam 9, the polarized light component in the direction perpendicular to the polarized light component 26 is an ordinary ray, so almost 100% is diffracted like the polarized light component indicated by the arrow 29. Becomes
Therefore, the polarization of the detection beams 10A and 10B is
The polarization components 28 and 29 are combined as indicated by an arrow 30. Here, as the diffraction efficiency of the linearly polarized light orthogonal to the polarization direction of the outgoing beam of the light source is higher than the diffraction efficiency of the outgoing beam 8 of the light source, the apparent Kerr rotation angle θ ′ becomes larger. Further, when all the linearly polarized light orthogonal to the polarization direction of the outgoing beam 8 of the light source is diffracted, the apparent Kerr rotation angle θ ′ becomes the largest. FIG. 5 shows the structure of the composite prisms 6A and 6B. Since the compound light prisms 6A and 6B have the same structure and have the same action on the detection beams 10A and 10B, the description will be given with reference to the same FIG. The polarization component 30 of the detection beam 10A or 10B is rotated by 45 degrees by the half-wave plate 34. Dotted line 3 shown in FIG.
1 indicates a direction of 45 degrees, and an arrow 30 indicates that the polarization component 30 shown in FIG. 4A is rotated by 45 degrees. The polarization component 30 is separated into a p-polarization component 32 and an s-polarization component 33 by the polarization separation film 35 shown in FIG. The p-polarized component 32 becomes the beam 11A or the beam 11B, and the s-polarized component 33 becomes the beam 12A or the beam 12B, and is reflected by the reflecting surface 36. FIG. 6 shows the configuration of the photodetectors 7A and 7B, and the photodetectors 7A and 7B have the same structure. The beam 11A or the beam 11B is received by the three-division photodetection elements 37, 38, 39, and the beam 12A or the beam 12B is received by the two-division photodetection elements 40, 41. Here, the three-division photodetector elements 37, 38, 3
The outputs of 9 are I ₁ , I ₂ , and I ₃ , and the two-split photodetector 4
If the outputs of 0 and 41 are I ₄ and I ₅ , I ₁ + I ₂ + I ₃
Is the intensity of the p-polarized component 32, and I ₄ + I ₅ is the intensity of the s-polarized component 48. Therefore, the magneto-optical signal is (I ₁ + I ₂ +
I ₃₎ - (obtained by calculation of I ₄ + I _5). When the magneto-optical disk 5 shifts from the focus position of the beam narrowed down by the focus lens 4, as shown in FIG.
The diameter of 1A or beam 11B changes. Therefore, the defocus detection signal can be obtained from the calculation signal of I ₁ − (I ₂ + I ₃ ) by the known defocus detection method. Further, the track deviation detection signal can be obtained from the calculation of I ₄ -I ₅ by a known push-pull method using diffraction by a groove. When the thickness of the birefringent substrate other than the grating groove 16 portion of the diffraction grating 3 is inappropriate, the phase difference generated between the ordinary ray and the extraordinary ray is not an integral multiple of the wavelength, and the beam incident as linearly polarized light is diffracted. When passing through the grating 3, it becomes elliptically polarized light, and S of the magneto-optical reproduction signal is obtained.
/ N ratio decreases. In this case, in the optical path of the reflected beam 9 of the magneto-optical disk, or in the detection beams 10A and 10B.
By inserting a compensating wave plate (retarder) in the optical path of
By compensating for the phase difference deviated from the integral multiple of the wavelength and setting the combined phase difference to the integral multiple of the wavelength, it is possible to prevent the S / N ratio from decreasing. The part indicated by the dotted line in FIG.
3 shows a LiNbO ₃ plate having the same thickness as the LiNbO ₃ substrate of the diffraction grating 3, and the direction of the optical axis is arranged orthogonal to the direction of the optical axis 15 of the diffraction grating 3. With such a configuration, the polarization component passing through the diffraction grating 3 as an ordinary ray passes through the retarder 13 of the LiNbO ₃ plate as an extraordinary ray, while the polarization component passing through the diffraction grating 3 as an extraordinary ray is , LiNbO ₃ plate passes through the retarder 13 as an ordinary ray. Therefore, the phase shifter 13 can compensate the phase difference generated in the diffraction grating 3. As described above, according to this embodiment, since the diffraction angle of the beam by the diffraction grating is smaller than the reflection angle (right angle) by the beam splitter, the composite prisms 6A, 6
Since B and the photodetectors 7A and 7B can be arranged near the semiconductor laser 1, the optical head can be downsized. Further, as the diffraction efficiency of the linearly polarized light orthogonal to the polarization direction of the outgoing beam of the light source is higher than the diffraction efficiency of the outgoing beam of the light source, the apparent Kerr rotation angle θ ′ increases, and the S / N ratio of the magneto-optical signal is increased. Can be improved.
Further, when all the linearly polarized light orthogonal to the polarization direction of the outgoing beam of the light source is diffracted, the apparent Kerr rotation angle θ ′ becomes the largest and the S / N ratio of the magneto-optical signal can be most improved. .

[0008]

According to the present invention, a magneto-optical disk is irradiated with a beam emitted from a light source as linearly polarized light, and the rotation of the polarization plane of the reflected light due to the Kerr effect is detected. Since the reproduced signal can be processed without separating the beam in the direction perpendicular to the optical path of the emitted beam of the semiconductor laser, the optical head is small and has a high S / N ratio, and the small and high performance optical information processing apparatus. Is obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a configuration of an optical head exemplified in an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a configuration of a diffraction grating of the optical head exemplified in the embodiment of the present invention.

FIG. 3 is an explanatory diagram showing the relationship between the groove depth and the phase difference of the diffraction grating of the optical head exemplified in the embodiment of the present invention.

FIG. 4 is an explanatory diagram of a polarization direction of the optical head exemplified in the embodiment of the invention.

FIG. 5 is an explanatory diagram showing a configuration of a composite prism of the optical head exemplified in the embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a configuration of a photodetector of the optical head exemplified in the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser 2 ... Collimating lens 3 ... Diffraction grating 4 ... Focus lens 5 ... Magneto-optical disk 6A, 6B ... Composite prisms 7A, 7B ... Photodetector 8 ... Emission beam 9 ... Reflection beam 10A, 10B ... Detection beam 11A, 11B, 12A, 12B ... Beam 13 ... Retardant 14 ... Refractive index ellipsoid 15 ... Optical axis (or C axis of crystal) 16 ... Lattice groove 17 ... Cut surface 18, 19 ... Linearly polarized light 20 ... Material with refractive index n 21 Diffraction grating 22 ... Convex optical path length 23 ... Recessed optical path length 24, 25 ... Straight lines 26, 27, 28, 29, 30 ... Arrows (polarization component) 31 ... Dotted line 32 ... P polarization component 33 ... S polarization component 34 ... Half-wave plate 35 ... Polarization separation film 36 ... Reflective surface 37, 38, 39 ... 3-division photodetector 40, 41 ... 2-division photodetector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masashi Suenaga Inventor, Masashi Suenaga 1-88, Tora, Ibaraki, Osaka

Claims (6)

[Claims] 1. A light source, an optical system for narrowing a beam emitted from the light source onto the magneto-optical information medium as linearly polarized light, a diffraction grating for separating a beam reflected by the magneto-optical information medium from the beam emitted from the light source, In an optical head including at least a photodetector that receives a separated beam diffracted and separated by a diffraction grating, the diffraction grating sets the separation ratio of the diffracted beam intensity to the incident beam intensity to be different for two orthogonal linearly polarized lights. An optical head that is characterized. 2. The optical head according to claim 1, wherein the diffraction grating has a polarization direction different from a polarization ratio of the first polarization when the polarization direction is not rotated by the magneto-optical information medium. An optical head characterized in that the separation ratio of the second polarized light which is orthogonal to is set to be larger. 3. The optical head according to claim 2, wherein the diffraction grating is configured to diffract and separate all the second polarized light. 4. The optical head according to claim 1, 2, or 3, wherein the diffraction grating is made of a birefringent material. 5. The optical head according to claim 4, wherein an element that gives a phase difference is arranged in the optical path of the reflected beam or the separated beam. 6. An optical information processing apparatus comprising an optical information processing apparatus using the optical head according to any one of claims 1 to 5.